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Open Access Publications from the University of California

Interface Design of Halide Perovskite and Metal Oxide Semiconductor for Optoelectronic Devices

  • Author(s): Sun, Pengyu
  • Advisor(s): Yang, Yang
  • Yablonovitch, Eli
  • et al.

Thin film optoelectronics, including solar cells and thin film transistors, has been investigated extensively in recent years. Thin film solar cells have attracted substantial attention due to their low cost, ease of fabrication, flexible nature and potential to achieve high power conversion efficiency. We witnessed a rapid rise of halide perovskite solar cells with power conversion efficiency reaching 22.7% within a few years, exceeding Cu(InxGa1-x)Se2 and CdTe. Perovskite materials have shown unique properties of defect tolerance and strong optical absorption, which makes the material a perfect candidate for photovoltaic applications. On the other hand, thin film transistors (TFT) have evolved at a fast pace with rapid industrialization. InGaZnO (IGZO) material has shown a quick adoption in flat panel display (FPD) backplanes since discovery in 2004. IGZO’s high mobility, good stability and uniformity enable the material to compete with traditional amorphous silicon (a-Si) or polycrystalline silicon (p-Si) technologies.

This thesis focuses on role of interface and fundamentals of optoelectronics for both perovskite solar cells and IGZO TFTs. In Chapter 2, I will discuss the theoretical background of optoelectronics where photoluminescence quantum efficiency (PLQE) plays an important role in perovskite thin film solar cells. I will also review the current progress in the field where people trying to optimize PLQE in perovskite material and future directions. In Chapter 3, I will demonstrate the impact of bottom contact on PLQE of CH3NH3PbI3 (MAPbI3) perovskite and its correlation with open circuit voltage of perovskite solar cells. It has been found that perovskite material shows improved crystal growth on NiOx bottom contact over PEDOT:PSS coated substrates. In Chapter 4, I will demonstrate the growth of formamidinium lead iodide (FAPbI3) single crystals with diameter up to 5 mm. The material shows phase transformation at elevated temperatures and a lower band gap compared with MAPbI3, making it more suitable for photovoltaics. FAPbI3 also shows high mobility compared with MAPbI3. In Chapter 5, I will demonstrate admittance spectroscopy for identification of defect states in MAPbI3 perovskite solar cells. Lastly, in Chapter 6, I will demonstrate stability enhancement for solution processed IGZO TFTs with top surface passivation by parylene coating. Solution processed IGZO material shows comparable performance metrics with traditional sputtered IGZO with respect to mobility, on-off ratio, subthreshold swing (SS), and threshold voltage. However, due to the nature of sol-gel process, the thickness of solution processed IGZO is limited and the material is prone to surface environment. I will demonstrate an effective way of passivation on solution processed IGZO with Parylene to minimize surface effect and the passivated TFT shows positive bias stress below 0.5 V drift in turn-on voltage.

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